Lipopolysaccharide (LPS) is the most abundant antigen on the surface of enterobacterial pathogens. Typically, LPS has three structural parts: i) Lipid A (also known as endotoxin), ii) core oligosaccharide, and iii) O-antigen. The latter is made up of repeating subunits of 3-6 sugars (depending on the serotype). Lipid A and core OS are relatively well conserved in one single enterobacterial species, however, their accessibility to antibodies are limited. On the other hand, O-antigens are highly accessible, but very diverse with respect to their structure (in E. coli there are ˜180 different O-types).
Antibodies against O-antigens are able to bind to the surface of E. coli, hence they are used both for diagnostics (e.g. O-typing for epidemiology studies) as well as are proposed as therapeutic measures. Nevertheless, given the huge structural variability, a broad spectrum protection with O-antigen specific antibodies is cumbersome.
Extraintestinal infections caused by E. coli are common causing significant morbidity and mortality. Multi-drug resistant (MDR) strains of E. coli that have emerged recently cause a significant proportion of E. coli infections.
Treatment options against these MDR strains are getting very limited as they have evolved resistance to most classes of clinically relevant antibiotics. Therefore, an alternative treatment option, e.g. passive immunization with monoclonal antibodies (mAbs) holds a great promise for the future.
In the past years a well-defined clonal lineage of MDR E. coli. the ST131-O25b:H4 has emerged causing approximately 10% of all extraintestinal E. coli infections and about half of the MDR E. coli infections (Peirano et al. Int J Antimicrob Agents 2010 April; 35(4):316-21; Rogers et al. J Antimicrob Chemother 2011 January; 66(1):1-14; Woodford et al. FEMS Microbiol Rev 2011 September; 35(5):736-55.). Strains belonging to this lineage show limited heterogeneity thus could be considered very similar with respect to antigenic repertoire. The vast majority of the strains belonging to this cluster express the O25b antigen and hence a specific gene (within the LPS synthesis locus) encoding for enzymes synthesizing this antigen is used for the identification of this clone (Clermont et al. J Antimicrob Chemother 2008 May; 61(5):1024-8.). Alternatively, agglutination with the O25 typing sera can be used, in spite that the O antigen of this lineage differ from the classical O25 antigen (hence it had been termed O25b) as suggested by genetic differences. However, the O25 typing sera can not distinguish between the non-MDR O25 and the MDR O25b clones.
Rogers et al. (J Antimicrob Chemother 2011 January; 66(1):1-14) describe the detection of the E. coli O25b-ST131 strain by three major characteristics, i.e. its serogroup (O25b), its phylogenetic group (B2) and its ST (ST131). Each of these characteristics is disclosed to aid detection. A variety of molecular techniques is described, i.e. MLST, PCR-based rapid detection methods, repetitive sequence PCR and PFGE. Polyclonal antisera (raised against an O25a strain) including a variety of immunoglobulins have been used to determine the O25 antigen, not differentiating subtypes.
Jadhav et al. (PLOS ONE 2011; 6(3): e18063) describe the virulence characteristics and genetic affinities of strains which were positive for the O25b subgroup that is linked to the B2-O25b-ST131-CTX-M-15 virulent/multiresistant type. Human clinical isolates were analyzed and classified into serotypes and virulence marker profiles were obtained. O25 positive strains were identified by serotyping using polyclonal antisera against O-antigens—O1 to O173. The O25 positive strains were further subjected to genotyping by an allele-specific PCR targeting the rfbO25b subgroup gene locus.
Mora et al. (Int. J. Antimicrobial Agents 2011; 37(1): 16-21) describe the emergence of some clonal groups among CTX-M-14 producing E. coli clinical isolates, among them O25b: H4-B2-ST131. O typing was done with specific O antisera (polyclonal).
Clermont et al. (J Antimicrob Chemother 2008 May; 61(5):1024-8) discloses an allele-specific pabB PCR assay specific for O25b ST131 E. coli. 
Szijarto et al, (FEMS Microbiol Lett 2012; 332:131-6) describe molecular typing of E. coli strain isolates based on the core structure of the LPS molecule. The core type of the isolates was determined by PCR using primers targeting genes in the core operon and specific to R1-4 and K-12 core types, respectively.